Eggplant polyphenol oxidase multigene family: cloning, phylogeny, expression analyses and immunolocalization in...

Though polyphenol oxidase (PPO) genes from tomato and potato have been extensively studied, information about PPO genes in eggplant (Solanum melongena) is lacking. The main objective of this study is to understand the structural and functional aspects of eggplant PPO genes. Six eggplant PPO genes (SmePPO1-6) cloned by RACE and genome walking were found to be intronless and correspond to eight eggplant unigenes. Comprehensive sequence analyses indicated that the eggplant PPO genes exhibit considerable variation in the transit peptide regions, copper-binding domains and UTRs, and fall into two distinct structural classes. Further, PPO gene members appear to exist in clusters on eggplant chromosome 8 as seen in the case of tomato and potato PPOs. During normal growth and development, SmePPO1 and 2 are expressed in roots, whereas the transcript levels of all the eggplant PPO genes vary considerably in leaves, flowers and fruits. SmePPO1 was expressed in Escherichia coli as a GST fusion protein, and immunoblot using rabbit polyclonal antiserum to GST-SmePPO1 detected a major protein band (~70 kDa) and a minor band (~67 kDa) in eggplant fruit extract. Tissue printing indicated the predominant presence of PPO in the exocarp and the areas surrounding the seeds in the mesocarp of eggplant fruits. Immunolocalization of PPOs in eggplant infested with shoot-and-fruit borer revealed localization of the PPO at the site of infection in tender shoots and fruits, and further inside the mature tissues. The upregulation of eggplant PPO gene transcripts following mechanical injury shows that all the genes except SmePPO2 are induced in the fruit over 6h. On the contrary, the transcripts of SmePPO2 and PPO3 are not detectable in the stem, and expression seems to be prominent over a 2h period for SmePPO1 and SmePPO4-6. Our results show that eggplant PPO genes are structurally different, and are differentially expressed in various tissues of eggplant indicating their functional diversity.